A silica powder has been used as a raw material of a filler for a plastic package, such as IC, LSI and the like, or for a raw material powder of a quartz glass crucible for pulling up a silicon single crystal of a semiconductor material. If impurities are contained in these silica powders, a semiconductor product is bad influenced so that the purified silica powder having few impurities is required.
As for a purification method of the silica powder, for example, Japanese Patent Application Laid Open No. 06-40713 discloses a method including: introducing gas containing chlorine or a chlorine compound into a silica powder at a temperature of 1000 to 1500° C. so as to fluidize the silica powder; and dehydrating the fluidized silica powder. Further, Japanese Patent Application Laid Open No. 08-290911 discloses a method including: continuously supplying a quarts powder to a rotary kiln; introducing a mixed gas of hydrogen chloride and chlorine into the kiln so as to contact to the quarts powder; and chloridizing an alkali metal contained in the quarts powders so as to be volatilized and removed. Furthermore, Japanese translation of PCT international application No. 2002-544102 discloses a method including: fluidizing a silica powder by chlorine containing treatment gas so as to remove metal impurities.
On the other hand, Japanese Patent Kokoku No. 07-14822 discloses an electrolytic purification method including: applying DC high voltage to a solid quartz glass so as to move alkali metals or the like in the glass on the cathode side; and purifying the quartz glass. Further, Japanese Patent Application Laid Open No. 2004-307222 discloses an example in which an electrolytic purification is applied to a quartz glass crucible. Furthermore, Japanese Patent Application Laid Open No. 2003-119018 discloses a purification method of a quarts powder including: applying high voltage to a quartz powder so as to charge impurity particles adsorbed on the quartz powder; and electrostatic-separating the quartz powder.
In the conventional methods for fluidizing a silica powder by a purification gas so as to remove impurities, chlorine gas is used as the purification gas in many methods, and gas added with hydrogen is also used in many methods. However, although the method using hydrogen gas can purify for a short time, there are problems that controlling of gas concentration is difficult since hydrogen gas has dangerous of an explosive reaction, and handling of the hydrogen gas is also difficult. Further, it is necessary to set the treatment temperature high such as about 1300° C. or more in order to increase a reaction rate.
On the other hand, although the method using chlorine gas or hydrogen chloride gas as the purification gas has high effect for removing impurities, it needs a long time for the purification. In addition, in order to carry out the purification within a practical time, the purification temperature must be set a high temperature of 1250° C. or more so as to obtain high purification efficiency. Thus, there is a problem in productivity. Further, the conventional electrolytic purification method also needs long treatment time.